Variable valve mechanism

ABSTRACT

Disclosed is a variable valve mechanism  10  for changing the lift amount and operating angle of an internal combustion engine valve disc  12 . The variable valve mechanism comprises a first cam  54 , which rotates in accordance with crankshaft rotation; a transmission member  24, 38  that includes a second cam  32, 34 , which oscillates in synchronism with the rotation of the first cam  54  and transmits the force exerted by the first cam  54  to the valve disc  12 ; a control shaft  40 , which is adjusted for a predetermined rotation position; an adjustment mechanism  36, 38  for varying the lift amount and operating angle of the valve disc  12  by changing the oscillation range of the transmission member  24, 38  in accordance with the rotation position of the control shaft  40 ; a lost motion spring  60  for pressing the transmission member  24, 38  toward the first cam  54  to ensure that the transmission member  24, 38  remains coupled to the first cam  54 ; and an assist spring  64  for pressing the transmission member  24, 38  in resistance to the force exerted by the lost motion spring  60.

TECHNICAL FIELD

The present invention relates to a variable valve mechanism, and moreparticularly to an internal combustion engine variable valve mechanism,which is capable of changing the operating angle and lift amount of avalve that opens/closes in synchronism with camshaft rotation.

BACKGROUND ART

A conventional variable valve mechanism is disclosed. The conventionalvariable valve mechanism is capable of changing the lift amount of avalve disc that opens/closes in synchronism with camshaft rotation. Thevariable valve mechanism disclosed, for instance, by Japanese PatentLaid-Open No. Hei 7-63023 is capable of changing the lift amount of avalve disc in accordance with the rotation position of an eccentricshaft. In this variable valve mechanism, a compression spring (lostmotion spring) is used to push a rocker lever, which is provided with aroller, in order to ensure that a roller whose contact with a cam variesin position with the eccentric shaft rotation position is pressedagainst the cam. When this variable valve mechanism is employed, thecompression spring works to ensure that the cam is in mechanical contactwith the roller at all times.

In the conventional mechanism disclosed by Japanese Patent Laid-Open No.Hei 7-63023, however, the compression spring coordinates with a valvespring to press the roller toward the cam. As a result, the eccentricshaft receives a force that is applied in a fixed direction.Consequently, the required drive torque of an actuator for eccentricshaft rotation increases, thereby lowering the responsiveness of avariable valve or increasing the power consumption.

Another variable valve mechanism disclosed, for instance, by JapanesePatent Laid-Open No. Hei 7-293216 is capable of changing the lift amountof a valve disc of an internal combustion engine. This variable valvemechanism includes a mechanical device that is positioned between thevalve disc and cam to change the lift amount. This mechanical deviceincreases the lift amount of the valve disc when a control shaft rotatesin a certain direction, and decreases the lift amount of the valve discwhen the control shaft rotates in another direction. When thismechanical device is employed, the lift amount of the valve disc can bearbitrarily changed by rotating the control shaft as appropriate.

The valve disc of an internal combustion engine is generally providedwith a valve spring, which pushes the valve disc in the valve closingdirection. Therefore, when the conventional variable valve mechanismopens the valve disc, the valve spring's reactive force is exerted onthe mechanical device between the valve disc and cam. The greater thelift amount for the valve disc, the greater the reactive force.

The mechanical device described above is dynamically stabler when thereactive force exerted on the mechanical device is small than when thereactive force exerted on the mechanical device is increased with anincrease in the lift of the valve disc. Therefore, the mechanical deviceis generally likely to change its state to decrease the lift amount. Inother words, it is likely that a reactive force for changing themechanical device state to a state corresponding to a small lift will betransmitted to the above-mentioned control shaft.

If the above reactive force is transmitted to the control shaft tochange the control shaft status, an appropriate lift amount cannot bemaintained for the valve disc. Therefore, this type of variable valvemechanism needs a mechanism for maintaining the control shaft statusconstant without regard to the valve spring's reactive force.

The control shaft of the conventional variable valve mechanism disclosedby Japanese Patent Laid-Open No. Hei 7-293216 is driven by a motor via agear mechanism. This gear mechanism includes a worm gear, which isinstalled over a motor rotation shaft, and a worm wheel, which mesheswith the worm gear. The gear mechanism, which includes the worm gear andworm wheel, provides high normal efficiency and low inverse efficiencydue to a great friction force exerted between the worm gear and wormwheel and a great gear ratio between them.

The above gear mechanism makes it possible to transmit a motor-generatedtorque to the control shaft with high efficiency and properly preventthe input to the control shaft from being transmitted to the motor.Therefore, the above conventional variable valve mechanism canaccurately control the control shaft status without being affected bythe valve spring. As a result, it is possible to accurately control thelift amount of the valve disc.

However, when the lift amount of the valve disc in the conventionalvariable valve mechanism disclosed by Japanese Patent Laid-Open No. Hei7-293216 is to be increased, it is necessary to rotate the control shaftin resistance to a reactive force for decreasing the lift amount. Morespecifically, it is necessary to rotate the control shaft in thedirection of increasing the lift amount in resistance to the valvespring's reactive force for decreasing the lift amount.

To meet the above requirements, it is necessary that the motor generatea great driving force. As a result, a motor cost increase, powerconsumption increase due to motor use, motor mountability deteriorationdue to structural expansion, and various other problems arise. Further,if such a great force is exerted on the control shaft, the control shaftmay significantly become distorted. In addition, the transmission ofsuch a great force increases the gear-to-gear contact load, therebyaccelerating the wear of gears.

The present invention has been made to solve the above problems. It isan object of the present invention to provide an internal combustionengine variable valve mechanism for changing the lift amount andoperating angle of a valve that opens/closes in synchronism withcamshaft rotation, and reduce the required load on a variable valve.

DISCLOSURE OF INVENTION

According to a first aspect of the present invention, a variable valvemechanism for changing the lift amount and operating angle of aninternal combustion engine valve disc comprises a first cam, whichrotates in accordance with crankshaft rotation; a transmission memberthat includes a second cam, which oscillates in synchronism with therotation of the first cam and transmits the force exerted by the firstcam to the valve disc; a control shaft, which is adjusted for apredetermined rotation position; an adjustment mechanism for varying thelift amount and operating angle of the valve disc by changing theoscillation range of the transmission member in accordance with therotation position of the control shaft; a lost motion spring forpressing the transmission member toward the first cam to ensure that thetransmission member remains coupled to the first cam; and an assistspring for pressing the transmission member in resistance to the forceexerted by the lost motion spring.

Since the assist spring is employed to press the transmission member inresistance to the force exerted by the lost motion spring, it ispossible to reduce the force that is exerted on the transmission memberby the lost motion spring. Therefore, it is easy to change thetransmission member oscillation range. Consequently, it is possible toreduce the control shaft drive torque for oscillation range changes. Asa result, the responsiveness of a variable valve improves, making itpossible to instantly change the lift amount and operating angle.Further, the control shaft drive torque decreases, making it possible touse a smaller-size actuator for driving the control shaft and minimizethe actuator current consumption.

According to a second aspect of the present invention, there is providedthe variable valve mechanism, which is improved as described above,wherein the lost motion spring presses the transmission member in thedirection of changing the lift amount and operating angle of the valvedisc from a great lift/great operating angle side to a small lift/smalloperating angle side; and wherein the force exerted on the transmissionmember by the assist spring increases with a decrease in the lift amountand operating angle of the valve disc.

When the force exerted on the transmission member by the lost motionspring changes from a great lift/great operating angle side to a smalllift/small operating angle side, the force exerted on the transmissionmember by the assist spring increases with a decrease in the liftamount/operating angle. It is therefore possible to reduce the controlshaft drive torque particularly when the variable valve is operated on asmall lift/small operating angle side.

According to a third aspect of the present invention, there is providedthe variable valve mechanism, which is improved as described above,further comprising a valve spring for pressing the valve disc toward thetransmission member, wherein the assist spring presses the transmissionmember in resistance to the valve spring's force that is exerted on thetransmission member via the valve disc.

Since the force exerted by the assist spring resists the force exertedby the valve spring, the force exerted on the transmission member by thevalve spring decreases. Therefore, the oscillation range of thetransmission member can easily be changed. Consequently, it is possibleto reduce the control shaft drive torque for oscillation range changes.

According to a fourth aspect of the present invention, there is providedthe variable valve mechanism, which is improved as described above,further comprising an actuator for generating a driving force forchanging the rotation position of the control shaft and a gear mechanismthat is positioned between the actuator and the control shaft, wherein aplurality of the transmission members, which are provided for the valvediscs of various cylinders, are coupled to the common control shaft;wherein the force exerted by the lost motion spring, the force exertedby the assist spring, and the force exerted by the valve spring aretransmitted in the rotation direction of the control shaft via thetransmission member and the adjustment mechanism; and wherein theresultant force applied in the rotation direction of the control shaftby the lost motion spring, the assist spring, and the valve springdecreases with an increase in the distance from the gear mechanism asviewed in the length direction of the control shaft.

Since the resultant force applied in the rotation direction of thecontrol shaft decreases with an increase in the distance from the gearmechanism, the resultant force applied to various parts of the controlshaft decreases with a decrease in the rigidity of the various parts ofthe control shaft. As a result, the degree of control shaft torsion canbe minimized.

According to a fifth aspect of the present invention, there is providedthe variable valve mechanism, which is improved as described above,wherein the force exerted on the transmission member by the assistspring increases with an increase in the distance from the gearmechanism as viewed in the length direction of the control shaft.

Since the force exerted on the transmission member by the assist springincreases with an increase in the distance from the gear mechanism, theassist spring load increases with a decrease in the rigidity of a partof the control shaft. A part of the control shaft that is positionedaway from the gear mechanism is likely to become distorted or otherwisemisshaped due to the force received from the lost motion spring or valvespring. However, the force exerted on the control shaft by the lostmotion spring or valve spring is reduced by the assist spring.Therefore, the degree of control shaft torsion can be minimized.

According to a sixth aspect of the present invention, there is providedthe variable valve mechanism, which is improved as described above,wherein the force exerted on the transmission member by the lost motionspring decreases with an increase in the distance from the gearmechanism as viewed in the length direction of the control shaft.

Since the force exerted on the transmission member by the lost motionspring decreases with an increase in the distance from the gearmechanism, the lost motion spring load decreases with a decrease in therigidity of a part of the control shaft. A part of the control shaftthat is positioned away from the gear mechanism is likely to becomedistorted or otherwise misshaped due to the force received from the lostmotion spring or valve spring. However, the force exerted on thetransmission member by the lost motion spring decreases with an increasein the distance from the gear mechanism. Therefore, the degree ofcontrol shaft torsion can be minimized.

According to a seventh aspect of the present invention, a variable valvemechanism, which is capable of changing the operating angle and/or liftamount of an internal combustion engine valve disc, comprises a controlshaft whose status is controlled to change the operating angle and/orlift amount; an oscillation arm that is positioned between a cam andvalve disc to oscillate in synchronism with cam rotation and transmitthe force exerted by the cam to the valve disc; an adjustment mechanismfor changing the basic relative angle of the oscillation arm relative tothe valve disc in accordance with the status of the control shaft; anactuator for generating a driving force for changing the status of thecontrol shaft; a gear mechanism that is positioned between the actuatorand control shaft; and assist force generation means for applying anassist force to the gear mechanism in order to increase the operatingangle and/or lift amount.

Since the status of the control shaft is controlled, the basic relativeangle of the oscillation arm relative to the valve disc can be varied.As a result, the operating angle and/or lift amount of the valve disccan be varied. Further, the present invention can apply an assist forceto the gear mechanism, which is positioned between the actuator andcontrol shaft, in order to increase the operating angle and/or liftamount. In other words, the present invention can apply an assist forceto the gear mechanism for the purpose of offsetting an inevitable forcethat is applied in the direction of decreasing the operating angleand/or lift amount. Therefore, the present invention can decrease anoutput, which is to be generated by the actuator for the purpose ofincreasing the operating angle and/or lift amount, by an amountequivalent to the assist force.

According to an eighth aspect of the present invention, there isprovided the variable valve mechanism, which is improved as describedabove, wherein the gear mechanism includes a worm wheel and worm gear,which are interconnected so as to position the worm gear toward theactuator and the worm wheel toward the control shaft; and wherein theassist force is applied to the worm wheel or to a structure integralwith the worm wheel.

The assist force to be applied to the gear mechanism can be given to theworm wheel. When the worm gear is to be rotated in the direction ofincreasing the operating angle and/or lift amount in this instance, itis possible to decrease a friction force that is exerted between theworm gear and worm wheel. The gear mechanism, which comprises the wormgear and worm wheel, exhibits higher normal efficiency in a stationarystate when the coefficient of static friction is smaller. Therefore, thepresent invention makes it possible to operate the control shaft in thedirection of increasing the operating angle and/or lift amount by usinga sufficiently small force, beginning with actuator startup.

According to a ninth aspect of the present invention, there is providedthe variable valve mechanism, which is improved as described above,further comprising a lost motion spring for pressing the oscillation armtoward the cam to ensure that the oscillation arm remains mechanicallycoupled to the cam, wherein the oscillation arm moves in the directionof increasing the amount of lost motion spring deformation when thegeneration of a great operating angle and/or lift amount is requested.

With the force generated by the lost motion spring, it is possible toensure that the oscillation arm remains mechanically coupled to the cam.The lost motion spring generates a force in the direction of inhibitingthe oscillation arm from moving in the direction of increasing theoperating angle and/or lift amount. In the present invention, the assistforce exerted on the gear mechanism also works to offset the forceexerted by the lost motion spring. Therefore, the present inventionmakes it possible to changing the control shaft in the direction ofincreasing the operating angle and/or lift amount by applying a smallforce while using the lost motion spring, which has characteristicsdescribed above.

According to a tenth aspect of the present invention, there is providedthe variable valve mechanism, which is improved as described above,wherein a plurality of the oscillation arms provided for the valve discsof various cylinders are coupled to the common control shaft; andwherein the force exerted by the lost motion spring decreases with anincrease in the distance from the gear mechanism as viewed in the lengthdirection of the control shaft.

Since the force exerted on the transmission member by the lost motionspring decreases with an increase in the distance from the gearmechanism, the lost motion spring load decreases with a decrease in therigidity of a part of the control shaft. A part of the control shaftthat is positioned away from the gear mechanism is likely to becomedistorted or otherwise misshaped due to the force received from the lostmotion spring or valve spring. However, the force exerted on thetransmission member by the lost motion spring decreases with an increasein the distance from the gear mechanism. Therefore, the degree ofcontrol shaft torsion can be minimized.

According to an eleventh aspect of the present invention, a variablevalve mechanism for changing the lift amount and operating angle of aninternal combustion engine valve disc comprises a first cam, whichrotates in accordance with crankshaft rotation; a transmission memberthat includes a second cam, which oscillates in synchronism with therotation of the first cam and transmits the force exerted by the firstcam to the valve disc; a control shaft, which is adjusted for apredetermined rotation position; an adjustment mechanism for varying thelift amount and operating angle of the valve disc by changing theoscillation range of the transmission member in accordance with therotation position of the control shaft; a lost motion spring forpressing the transmission member toward the first cam to ensure that thetransmission member remains coupled to the first cam; and an assistspring for generating a force that resists the force exerted by the lostmotion spring.

Since the assist spring is provided to generate a force that resists theforce exerted by the lost motion spring, it is possible to reduce theforce exerted by the lost motion spring. It is therefore possible toreduce the control shaft drive torque for transmission memberoscillation range changes. As a result, the responsiveness of a variablevalve improves, making it possible to instantly change the lift amountand operating angle. Further, the control shaft drive torque decreases,making it possible to use a smaller-size actuator for driving thecontrol shaft and minimize the actuator current consumption.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view illustrating the essential parts of avariable valve mechanism according to a first embodiment of the presentinvention;

FIG. 2 is an exploded perspective view illustrating a first arm memberand second arm member, which constitute the variable valve mechanismshown in FIG. 1;

FIGS. 3A and 3B illustrate a small lift operation that is performed bythe variable valve mechanism according to the first embodiment of thepresent invention;

FIGS. 4A and 4B illustrate a great lift operation that is performed bythe variable valve mechanism according to the first embodiment of thepresent invention;

FIG. 5 is a schematic diagram illustrating the essential parts of thevariable valve mechanism according to the first embodiment of thepresent invention;

FIGS. 6A and 6B are schematic diagrams illustrating the status of anassist spring that prevails when a control shaft rotation angle θ_(C) ischanged;

FIG. 7 is a schematic diagram illustrating an assist spring layout andcontrol shaft rotation mechanism;

FIG. 8 is a characteristic diagram indicating that a motor drive torqueis reduced by the use of an assist spring;

FIG. 9 is a schematic diagram illustrating a variable valve mechanismaccording to a second embodiment of the present invention;

FIG. 10 is a schematic diagram illustrating a lost motion spring that ismade of a torsion spring;

FIGS. 11A, 11B, and 11C illustrate the overall configuration of avariable valve mechanism according to a third embodiment of the presentinvention;

FIG. 12 illustrates the relationship between the normal efficiency of agear mechanism, which comprises a worm gear and worm wheel, and theirinstantaneous rotation speed; and

FIGS. 13A and 13B illustrate a lubricating oil flow path that is used ina variable valve mechanism according to a fourth embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings. Like elements in thedrawings are designated by like reference numerals and will not bedescribed repeatedly. The present invention is not limited to theembodiments described below.

First Embodiment

FIG. 1 is a perspective view illustrating the essential parts of avariable valve mechanism 10 according to a first embodiment of thepresent invention. The variable valve mechanism shown in FIG. 1 is amechanism for driving an internal combustion engine valve disc. It isassumed that each cylinder in an internal combustion engine is equippedwith two intake valves and two exhaust valves. The configuration shownin FIG. 1 functions as a mechanism for driving two intake valves or twoexhaust valves that are provided for a cylinder.

The configuration shown in FIG. 1 includes two valve discs 12, whichfunction as intake valves or exhaust valves. A valve stem 14 is fastenedto each valve disc 12. The end of the valve stem 14 is in contact with apivot that is mounted on one end of a rocker arm 16. The valve stem 14is pressed by a valve spring 62, which will be described later. Therocker arm 16 is pressed upward by the valve stem 14, which is pressedby the valve spring 62. The other end of the rocker arm 16 is supportedby a hydraulic lash adjuster 18 in a turnable manner. When the verticalposition of the rocker arm 16 is automatically adjusted by means ofhydraulic pressure, a tappet clearance can be automatically adjusted bythe hydraulic lash adjuster 18.

A roller 20 is provided at the center of the rocker arm 16. Anoscillation arm 22 is positioned over the roller 20. The structure ofthe oscillation arm section will now be described with reference to FIG.2.

FIG. 2 is an exploded perspective view illustrating a first arm member24 and a second arm member 26. Both the first arm member 24 and secondarm member 26 are major component members within the configuration shownin FIG. 1. As shown in FIG. 2, the aforementioned oscillation arm 22 isa part of the first arm member 24.

In other words, the first arm member 24 includes two oscillation arms 22and a roller contact surface 28 which are formed integrally, as shown inFIG. 2. The roller contact surface 28 is sandwiched between theoscillation arms 22. The two oscillation arms 22 are providedrespectively for the two valve discs 12 and both in contact with theaforementioned roller 20 (see FIG. 1).

The first arm member 24 is provided with a bearing section 30, which isa through-hole in the two oscillation arms 22. The surface of eachoscillation arm 22 that comes into contact with the roller 20 isprovided with a concentric section 32 and a pushing pressure section 34.The concentric section 32 is provided so that the surface in contactwith the roller 20 is concentric with the bearing section 30. Meanwhile,the pushing pressure section 34 is provided so that its leading end ispositioned farthest from the center of the bearing section 30.

The second arm member 26 is equipped with a non-oscillation section 36and an oscillation roller section 38. The non-oscillation section 36 hasa through-hole into which a control shaft 40 is inserted. Further, alock pin 42 is inserted into the non-oscillation section 36 and controlshaft 40 to lock the positional relationship between the non-oscillationsection 36 and control shaft 40. Therefore, the non-oscillation section36 and control shaft 40 function as a single structure.

The oscillation roller section 38 has two sidewalls 44. The sidewalls 44are coupled to the non-oscillation section 36 via a rotation shaft 46 ina freely turnable manner. A cam contact roller 48 and a slide roller 50are positioned between the two sidewalls 44. The cam contact roller 48and slide roller 50 can turn freely while they are sandwiched betweenthe sidewalls 44.

The aforementioned control shaft 40 is a member that is retained by thebearing section 30 of the first arm member 24 in a turnable manner. Inother words, the control shaft 40 is a member that should be integralwith the non-oscillation section 36 while it is retained by the bearingsection 30 in a rotatable manner. To meet this requirement, thenon-oscillation section 36 (that is, the second arm member 26) ispositioned between the two oscillation arms 22 of the first arm member24 before being fastened to the control shaft 40. After this positionaladjustment is made, the control shaft 40 is allowed to penetrate throughthe two bearing sections 30 and non-oscillation section 36. The lock pin42 is then inserted to secure the control shaft 40 and non-oscillationsection 36. As a result, the first arm member 24 is allowed to freelyturn on the control shaft 40. Further, the non-oscillation section 36becomes integral with the control shaft 40 to form a mechanism in whichthe oscillation roller section 38 can oscillate in relation to thenon-oscillation section 36.

When the first arm member 24 and second arm member 26 are assembledtogether as described above, the slide roller 50 of the oscillationroller section 38 can come into contact with the roller contact surface28 of the first arm member 24 as far as predefined conditions aresatisfied by the relative angle between the first arm member 24 andcontrol shaft 40, that is, the relative angle between the first armmember 24 and non-oscillation section 36. When the first arm member 24turns on the control shaft 40 within a range within which the predefinedconditions are met while the contact between the slide roller 50 androller contact surface 28 is maintained, the slide roller 50 can rollalong the roller contact surface 28. The variable valve mechanismaccording to the present embodiment opens/closes the valve disc 12 whilethe slide roller 50 rolls along the roller contact surface 28. The valvedisc operation will be described in detail later with reference to FIGS.3A, 3B and FIGS. 4A, 4B.

FIG. 1 shows the first arm member 24, second arm member 26, and controlshaft 40, which are assembled together in the sequence described above.In the resultant state, the positions of the first arm member 24 andsecond arm member 26 are regulated by the position of the control shaft40. The control shaft 40 is fastened to a cylinder head or other fixedmember via a bearing, which is not shown, in such a manner as to meetthe aforementioned conditions, that is, to bring the roller 20 of therocker arm 16 into contact with the oscillation arm 22.

As described later, an actuator (motor 66) is coupled to the controlshaft 40. This actuator can pivot the control shaft 40 within apredetermined angular range. FIG. 1 shows a state in which the rotationangle of the control shaft 40 is adjusted by the actuator so as to meetthe aforementioned predefined conditions and bring the slide roller 50into contact with the roller contact surface 28.

The variable valve mechanism 10 according to the present embodiment isequipped with a camshaft 52, which rotates in synchronism with acrankshaft. A cam 54, which is provided for each internal combustionengine cylinder, is fastened to the camshaft 52. In a state shown inFIG. 1, the cam 54 is in contact with the cam contact roller 48 andregulates the upward motion of the oscillation roller section 38. Inother words, in the state shown in FIG. 1, the roller contact surface 28of the first arm member 24 is mechanically coupled to the cam 54 via thecam contact roller 48 of the oscillation roller section 38 and the slideroller 50.

When, in the state described above, a cam nose applies pressure to thecam contact roller 48 during the rotation of the cam 54, the pressure istransmitted to the roller contact surface 28 via the slide roller 50.The slide roller 50 can continuously transmit the force exerted by thecam 54 to the first arm member 24 while rolling over the roller contactsurface 28. As a result, the first arm member 24 rotates around thecontrol shaft 40, causing the oscillation arm 22 to depress the rockerarm 16 and moving the valve disc 12 in the valve opening direction. Asdescribed above, the variable valve mechanism 10 operates the valve disc12 by transmitting the force exerted by the cam 54 to the roller contactsurface 28 via the cam contact roller 48 and slide roller 50.

The operation of the variable valve mechanism 10 according to the firstembodiment of the present invention will now be described with referenceto FIGS. 3A, 3B, 4A, and 4B. As described earlier, the variable valvemechanism 30 drives the valve disc 12 by mechanically transmitting theforce exerted by the cam 54 to the roller contact surface 28. To allowthe variable valve mechanism 10 to operate the valve disc 12 properly,it is necessary to ensure that the cam 54 is mechanically coupled to theroller contact surface 28 via the cam contact roller 48 and slide roller50. To meet this requirement, it is necessary to press the rollercontact surface 28, that is, the first arm member 24, toward the cam 54.A lost motion spring 60, which is shown in FIGS. 3A, 3B, 4A, and 4B, isused to press the roller contact surface 28 toward the cam 54. The valvespring 62 shown in FIGS. 3A, 3B, 4A, and 4B is used to press the valvedisc 12 and rocker arm 16 in the valve closing direction as describedearlier.

The upper end of the lost motion spring 60 is fastened to a cylinderhead or the like. The lower end of the lost motion spring 60 presses thetrailing end of the oscillation arm 22, which is opposite the side onwhich the roller contact surface 28 is provided. In this state,therefore, the lost motion spring 60 generates a force that lifts up theroller contact surface 28 of the oscillation arm 22 (a force forrotating the oscillation arm 22 counterclockwise around the controlshaft 40 in FIGS. 3A, 3B, 4A, and 4B). This force causes the rollercontact surface 28 to push the slide roller 50 upward and presses thecam contact roller 48 against the cam 54 (see FIGS. 1 and 2). As aresult, the variable valve mechanism 10 ensures that the cam 54 remainsmechanically coupled to the roller contact surface 28 as indicated inFIG. 1.

FIGS. 3A and 3B illustrate an operation that the variable valvemechanism 10 performs to give a small lift to the valve disc 12. Thisoperation is hereinafter referred to as a “small lift operation.” Morespecifically, FIG. 3A indicates that the valve disc 12 closes during asmall lift operation, and FIG. 3B indicates that the valve disc 12 opensduring a small lift operation.

In FIG. 3A, the symbol θ_(C) denotes a parameter that indicates therotation position of the control shaft 40. This parameter is hereinafterreferred to as the “control shaft rotation angle θ_(C).” For the sake ofsimplicity, it is defined that the control shaft rotation angle θ_(C) isan angle between the vertical and a straight line joining the center ofthe control shaft 40 to the center of the rotation shaft 46. In FIG. 4A,the symbol θ_(A) denotes a parameter that indicates the rotationposition of the oscillation arm 22. This parameter is hereinafterreferred to as the “arm rotation angle θ_(A).” For the sake ofsimplicity, it is defined that the arm rotation angle θ_(A) is an anglebetween the horizontal and a straight line joining the leading end ofthe oscillation arm 22 to the center of the control shaft 40.

In the variable valve mechanism 10, the rotation position of theoscillation arm 22, that is, the arm rotation angle θ_(A), is determinedby the position of the slide roller 50. The position of the slide roller50 is determined by the position of the rotation shaft 46 in theoscillation roller section 38 and the position of the cam contact roller48. Within a range within which the cam contact roller 48 is in contactwith the cam 54, the position of the slide roller 50 moves upward as therotation shaft 46 rotates counterclockwise in FIGS. 4A and 4B, that is,as the control shaft rotation angle θ_(C) decreases. In the variablevalve mechanism 10, therefore, the smaller the control shaft rotationangle θ_(C), the smaller the arm rotation angle θ_(A).

In the state shown in FIG. 3A, the control shaft rotation angle θ_(C) isvirtually minimized within a range within which the cam contact roller48 is in contact with the cam 54, that is, the cam 54 can regulate theupward motion of the cam contact roller 48. In the state indicated inFIG. 3A, therefore, the arm rotation angle θ_(A) is virtually minimized.The variable valve mechanism 10 is configured so that the approximatecenter of the concentric section 32 of the oscillation arm 22 is incontact with the roller 20 of the rocker arm 16 in the above instance.As a result, the valve disc 12 closes. The arm rotation angle θ_(A)prevailing in the above instance is hereinafter referred to as the“small lift reference arm rotation angle θ_(A0).” As described later,the rotation angle of the control shaft 40 is locked to a value that isselected by the actuator.

When the cam 54 rotates in the state shown in FIG. 3A, the cam contactroller 48 moves toward the control shaft 40 as it is pressed by the camnose as indicated in FIG. 3B. The distance between the slide roller 50and the rotation shaft 46 of the oscillation roller section 38 does notchange. Therefore, when the cam contact roller 48 approaches the controlshaft 40, the roller contact surface 28 is depressed by the slide roller50, which rolls over the roller contact surface 28. As a result, theoscillation arm 22 rotates in the direction of increasing the armrotation angle θ_(A) so that the point of contact between theoscillation arm 22 and roller 20 moves from the concentric section 32 tothe pushing pressure section 34.

When the pushing pressure section 54 comes into contact with the roller40 in accordance with the rotation of the oscillation arm 42, the valvedisc 12 moves in the valve opening direction in resistance to the forceexerted by the valve spring 62. The maximum lift amount is given to thevalve disc 12 when the arm rotation angle θ_(A) is maximized. When asmall lift operation is performed, the reference arm rotation angleθ_(A0) is set to a small value as described above. Therefore, themaximum value of the arm rotation angle θ_(A) prevailing during therotation of the cam 54 is relatively small for a small lift operation.The maximum arm rotation angle prevailing during a small lift operationis hereinafter referred to as the “small lift maximum arm rotation angleθ_(AMAX).” The maximum lift is given to the valve disc 12 when the armrotation angle θ_(A) is maximized so that the maximum arm rotation angleθ_(AMAX) prevails. As indicated in FIG. 3B, the variable valve mechanism10 is configured so that when the small lift maximum arm rotation angleθ_(AMAX) prevails, the point of contact between the roller 20 andoscillation arm 22 slightly moves into the pushing pressure section 34,thereby giving a slight lift to the valve disc 12. Therefore, when thesmall lift operation described above is performed, the variable valvemechanism 10 gives a small lift to the valve disc 12 in synchronism withthe rotation of the cam 54.

In the above instance, the period during which the force exerted by thecam 54 actually depresses the valve disc 12, that is, the period duringwhich the valve disc 12 is not closed due to the rotation of the cam 54(crank angular width), is relatively short (this period is hereinafterreferred to as the “operating angle”). Therefore, when a small liftoperation is performed, the variable valve mechanism 10 decreases boththe lift amount and operating angle of the valve disc 12. In such aninstance, a relatively small valve spring reactive force is exerted onthe oscillation arm 22 when the valve disc 12 opens.

FIGS. 4A and 4B illustrate an operation that the variable valvemechanism 10 performs to give a great lift to the valve disc 12. Thisoperation is hereinafter referred to as a “great lift operation.” Morespecifically, FIG. 4A indicates that the valve disc 12 closes during agreat lift operation, and FIG. 4B indicates that the valve disc 12 opensduring a great lift operation.

When a great lift operation is to be performed, the control shaftrotation angle θ_(C) is adjusted for a sufficiently great value asindicated in FIG. 4A. As a result, when a great lift operation isperformed, the arm rotation angle θ_(A) prevailing during a non-liftperiod, that is, the reference arm rotation angle θ_(A0), becomes asufficiently great value within a range within which the slide roller 50does not leave the roller contact surface 28. The variable valvemechanism 10 is configured so that the point of contact between theoscillation arm 22 and roller 20 is positioned at the end of theconcentric section 32 when the reference arm rotation angle θ_(A0)prevails. Therefore, the valve disc 12 also remains closed when a greatlift operation is performed.

When the cam 54 rotates in the state shown in FIG. 4A, the cam contactroller 48 is pressed by the cam nose as indicated in FIG. 4B. Theoscillation arm 22 then rotates in the direction of increasing the armrotation angle θ_(A). As a result, the point of contact between theoscillation arm 22 and roller 20 moves from the concentric section 32 tothe pushing pressure section 34, thereby moving the valve disc 12 in thevalve opening direction in resistance to the reactive force exerted bythe valve spring 62. When a great lift operation is performed, thereference arm rotation angle θ_(A0) becomes a great value as describedabove. Therefore, the maximum arm rotation angle θ_(AMAX), which ariseswhen the cam 54 rotates, also becomes a great value. The variable valvemechanism 10 is configured so that when the maximum arm rotation angleθ_(AMAX) arises, the point of contact between the roller 20 andoscillation arm 22 is sufficiently inserted into the pushing pressuresection 34 as indicated in FIG. 4B. Therefore, while the great liftoperation described above is being performed, the variable valvemechanism 10 can give a great lift and great operating angle to thevalve disc 12 in synchronism with the rotation of the cam 54 asindicated in FIG. 4B. Since the lift amount for the valve disc 12 isgreat in this instance, a relatively great valve spring reactive forceis exerted on the oscillation arm 22 when the valve disc 12 opens.

The reactive force, which is exerted by the valve spring 62 when thevalve disc 12 opens, presses the oscillation arm 22 in the direction ofdecreasing the arm rotation angle θ_(A). In other words, this reactiveforce moves the control shaft 40 in the direction of decreasing thecontrol shaft rotation angle θ_(C). In the variable valve mechanism 10,the reactive force generated by the valve spring 62 works to rotate thecontrol shaft 40 in the direction of decreasing the operating angle andlift amount.

In the variable valve mechanism 10, the force of the lost motion spring60 and the aforementioned reactive force of the valve spring 62 are bothexerted on the control shaft 40. This exerted force by the lost motionspring 60 also works in the direction of decreasing the control shaftrotation angle θ_(C), that is, in the direction of decreasing theoperating angle and lift amount of the valve disc 12, as is the casewith the reactive force of the valve spring 62.

The force exerted by the lost motion spring 62 increases with anincrease in the amount of its deformation. In the present embodiment,the amount of deformation increases as the first arm member 24 rotatesin the direction of increasing the arm rotation angle θ_(A). Further,the present embodiment is configured so that the arm rotation angleθ_(A) increases with an increase in the lift amount generated for thevalve disc 12. When the valve disc 12 exhibits the maximum lift during agreat lift operation in the variable valve mechanism 10, the lost motionspring 62 generates a particularly great force (see the status of thelost motion spring 60 in FIG. 4B). As a result, a particularly greattorque is applied to operate the control shaft 40 in the direction ofdecreasing the lift amount.

As described above, the variable valve mechanism 10 according to thepresent embodiment changes the control shaft rotation angle θ_(C) tochange the reference arm rotation angle θ_(A0), thereby changing theoperating angle and lift amount to be given to the valve disc 12.

The essential parts of the variable valve mechanism 10 according to thepresent embodiment will now be described with reference to FIG. 5. Asdescribed earlier, the lost motion spring 60 generates a force forlifting up the roller contact surface 28 of the oscillation arm 22. Asindicated in FIG. 5, an upward force, which is generated by the valvespring 62, is exerted on the valve stem 14. The valve stem 14, whichreceives the force of the valve spring 62, pushes the rocker arm 16upward. When the roller 20 of the rocker arm 16 is in contact with thepushing pressure section 34 depending on the rotation position of thecam 54, the force of the valve spring 62 also works to lift up theroller contact surface 28.

Therefore, the force of the lost motion spring 60 and the force of thevalve spring 62 both works in the same direction as the rotationdirection of the oscillation arm 22. These two springs operate so thatthe force exerted on the oscillation arm 22 works in the direction oflifting up the roller contact surface 28 (in the direction of rotatingthe oscillation arm 22 counterclockwise in FIG. 5). The force forlifting up the roller contact surface 28 is transmitted to thenon-oscillation section 36 via the slide roller 50, oscillation rollersection 38, and rotation shaft 46. The non-oscillation section 36 andthe control shaft 40, which is integral with the non-oscillation section36, then receive a force for counterclockwise rotation around thecontrol shaft 40 in FIG. 5.

Therefore, when the control shaft 40 rotates in the direction ofdecreasing the control shaft rotation angle θ_(C), that is, when thecontrol shaft 40 rotates from the great lift operation side to the smalllift operation side, the direction in which the force of the lost motionspring 60 and the force of the valve spring 62 affect the rotation ofthe control shaft 40 is the same as the rotation direction of thecontrol shaft 40. Therefore, the torque for rotating the control shaft40 is relatively small.

When, on the other hand, the control shaft 40 rotates from the smalllift operation side to the great lift operation side, the direction inwhich the force of the lost motion spring 60 and the force of the valvespring 62 affect the rotation of the control shaft 40 is opposite therotation direction of the control shaft 40. Therefore, a great torque isrequired for rotating the control shaft 40.

Under the above circumstances, the variable valve mechanism 10 accordingto the present invention includes an assist spring 64, which exerts aforce in the direction opposite the direction in which the force of thelost motion spring 60 and the force of the valve spring 62 are exerted,as shown in FIG. 5. The assist spring 64 comprises a torsion spring thatis appropriate for space saving. When the assist spring 64 iscompressed, one of its ends comes into contact with an upper surfacenear the rotation shaft 46 of the non-oscillation section 36. The otherend is fixed. Thus, the force of the assist spring 64 works in thedirection of rotating the control shaft 40 clockwise in FIG. 5.Consequently, the force exerted on the control shaft 40 by the assistspring 64 is oriented in the direction opposite the direction in whichthe force of the lost motion spring 60 and the force of the valve spring62 affect the rotation of the control shaft 40.

The torque required for rotating the control shaft 40 clockwise in FIG.5 can then be reduced. Thus, the control shaft drive torque requiredparticularly for switching from the small lift operation side to thegreat lift operation side can be reduced. It is therefore possible todrive the control shaft 40 quickly. Further, since the drive torque isreduced, the power consumption for the actuator, which drives thecontrol shaft 40, can be minimized.

FIGS. 6A and 6B are schematic diagrams illustrating the status of theassist spring 64 that prevails when the control shaft rotation angleθ_(C) is changed. FIG. 6A shows a case where the control shaft rotationangle θ_(C) is set for the small lift operation side (small operatingangle side), whereas FIG. 6B shows a case where the control shaftrotation angle θ_(C) is set for the great lift operation side (greatoperating angle side).

When the control shaft rotation angle θ_(C) is set for the small liftoperation side as indicated in FIG. 6A, the control shaft rotation angleθ_(C) is minimized so that the assist spring 64 is compressed to themaximum extent. In this state, the force of the assist spring 64 becomesmaximized and works to rotate the control shaft 40 clockwise. Therefore,the force of the lost motion spring 60 and the force of the valve spring62 are offset. Thus, the drive torque required for rotating the controlshaft 40 toward the great lift operation side (great operating angleside) decreases. Consequently, it is possible to quickly switch from thesmall operating angle/small lift state to the great operatingangle/great lift state when the vehicle is to be started or acceleratedin an idling or steady driving state of engine. As a result, thedrivability prevailing at the time of vehicle startup/acceleration canbe improved.

When, on the other hand, the control shaft rotation angle θ_(C) is setfor the great lift operation side as indicated in FIG. 6B, the controlshaft rotation angle θ_(C) is maximized so that the force exerted on thecontrol shaft 40 by the assist spring 64 is reduced. Further, the forceof the lost motion spring 60 and the force of the valve spring 62 workin the direction of rotating the control shaft 40 counterclockwise.Therefore, the control shaft drive torque for switching from the currentstate to the small lift operation side is minimized. As a result, theoperating angle/lift amount can be quickly changed with a small drivetorque on the great lift operation side as well.

FIG. 7 is a schematic diagram illustrating an assist spring layout andcontrol shaft rotation mechanism. As indicated in FIG. 7, the variablevalve mechanism 10 includes a mechanism for rotating the control shaft40. FIG. 7 shows two cylinders (cylinders #1 and #2). Each cylinder isequipped with two valve discs 12, which serve as intake or exhaustvalves.

As shown in FIG. 7, the control shaft 40 is provided with a spring guide66, which retains the assist spring 64. The spring guide 66 ispositioned over the control shaft 40. The spring guide 66 comprises abar member or tubular member, which are shared by two adjacentcylinders, and is fastened to a spring guide head 68. The spring guidehead 68 is fastened, for instance, to the cylinder head or a cap thatsupports the control shaft 40 in a rotatable manner.

Two cylinder assist springs 64 are wound around the spring guide 66. Oneend of each assist spring 64 is fixed by inserting it into a hole in aspring guide cap 68. The other end of each assist spring 64 is incontact with the non-oscillation section 36 of the second arm member 26and used to press the non-oscillation section 36.

The spring guide cap 68 is provided with a slit 68 a, a bolt 70 isinserted into the spring guide cap 68. The bolt 70 is fastened, forinstance, to the cylinder head or a cap that supports the control shaft40 in a rotatable manner. This ensures that the spring guide cap 68 isfastened, for instance, to the cylinder head, and that the spring guide66 is fastened to the spring guide cap 68.

The end of the control shaft 40 is provided with a worm wheel 72. Amotor 66 for driving the control shaft 40 is installed near the wormwheel 72. A motor shaft 74 for the motor 66 is provided with a worm gear76. The worm wheel 72 is in engagement with the worm gear 76. Therefore,when the motor shaft 74 rotates, the engagement between the worm gear 76and worm wheel 72 causes the control shaft 40 to rotate. A positionsensor 78 is mounted on the end of the control shaft 40 to detect therotation angle of the control shaft 40.

In a mechanism for allowing the engagement between the worm wheel 72 andworm gear 76 to rotate the control shaft 40, the self-lock function of aworm gear mechanism is used as indicated in FIG. 7 to maintain therotation angle of the control shaft 40 as specified. In such a worm gearmechanism, gear tooth surfaces slide against each other. Therefore, thestatic friction coefficient for the gear tooth surfaces is great so thatthe contact load on the gear tooth surfaces significantly affects thedrive torque. Consequently, when only the forces of the lost motionspring 60 and valve spring 62 work in the rotation direction of thecontrol shaft 40, the contact load on the gear tooth surfaces increases,thereby increasing the torque for driving the worm gear 76. Since thepresent embodiment is provided with the assist spring 64, it minimizesthe contact load on the gear tooth surfaces of the worm wheel 72 andworm gear 76. It is therefore possible to considerably decrease thedrive torque of the control shaft 40, particularly the startup torque.

FIG. 8 is a characteristic diagram indicating that the drive torque ofthe motor 66 is reduced by the use of the assist spring 64. Thehorizontal axis of the diagram indicates the control shaft rotationangle θ_(C) (deg), whereas the vertical axis indicates the drive torqueof the motor 66. The characteristic curves indicated in FIG. 8 prevailwhen the control shaft 40 is rotated from the small lift operation sideto the great lift operation side.

The characteristic curve indicated by a broken line in FIG. 8 prevailswhen the assist spring 64 is not provided. In such a situation, only theforces of the lost motion spring 60 and valve spring 62 work in thedirection of rotating the control shaft 40. Therefore, the drive torquefor rotating the control shaft 40 from the small lift operation side tothe great lift operation side increases.

The characteristic curve indicated by a solid line in FIG. 8 prevailswhen the assist spring 64 is provided. In such a situation, the assistspring 64 offsets the forces of the lost motion spring 60 and valvespring 62. The drive torque of the control shaft 40 can therefore bereduced to approximately one-third to one-half. Even when the assistspring 64 is provided, the drive torque for switching from the greatlift operation side to the small lift operation side hardly increases.The reason is that a drive torque decrease, which is encountered whenthe assist spring 64 is provided, is mainly caused by a decrease in thecontact load on the gear tooth surfaces of the worm gear mechanism.Therefore, it is preferred that the force of the assist spring 64 beadequate for reducing the contact load on the gear tooth surfaces of theworm gear mechanism.

As described above, the first embodiment is provided with the assistspring 64, which exerts a force in the direction opposite the directionin which the forces of the lost motion spring 60 and valve spring 62 areexerted. It is therefore possible to considerably decrease the drivingforce for rotating the control shaft 40. The responsiveness for drivingthe control shaft 40 can then be enhanced to quickly change the valvelift amount and operating angle in accordance with operating conditions.Further, the contact load on the gear tooth surfaces of the worm gearmechanism for driving the control shaft 40 can be considerably decreasedto control the wear of the gear tooth surfaces. Furthermore, the size ofthe motor 76 for driving the control shaft 40 can be reduced to minimizethe power consumption of the motor 76.

In the first embodiment, which has been described above, the first armmember 24 and oscillation roller section 38 correspond to the“transmission member” according to the first or eleventh aspect of thepresent invention; the non-oscillation section 36 and oscillation rollersection 38 correspond to the “adjustment mechanism” according to thefirst or eleventh aspect of the present invention; the cam 54corresponds to the “first cam” according to the first or eleventh aspectof the present invention; and the concentric section 32 and pushingpressure section 34 correspond to the “second cam” according to thefirst or eleventh aspect of the present invention.

Second Embodiment

A second embodiment of the present invention will now be described. FIG.9 is a schematic diagram illustrating a variable valve mechanism 10according to the second embodiment. The second embodiment of thevariable valve mechanism 10 has the same basic configuration as thefirst embodiment.

As is the case with the first embodiment, each of cylinders #1 to #4 isprovided with the assist spring 64 for decreasing the drive torque ofthe control shaft 40. In the second embodiment, different force settingsare employed for the assist springs 64 in consideration of control shaftdeformation.

As described in conjunction with the first embodiment, the forces of thelost motion spring 60 and valve spring 62, which are exerted on thecontrol shaft 40, are oriented in the same rotation direction. Eachcylinder is provided with one lost motion spring 60 and two valvesprings 62. Therefore, the loads applied by these springs are imposed onthe control shaft 40, which is shared by the cylinders.

Therefore, when, for instance, the control shaft 40 is made of a thin,hollow pipe, the forces of the lost motion spring 60 and valve spring 62distort the control shaft 40, thereby causing the control shaft 40 todeform in the direction of rotation. In such an instance, the controlshaft 40 is locked by the worm gear mechanism to prevent it fromrotating. The rigidity of the control shaft 40 decreases with anincrease in the distance from the worm gear mechanism. Therefore, theamount of control shaft deformation increases with an increase in thedistance from the worm wheel 72.

As such being the case, the second embodiment is configured so that theforce of the assist spring 64 increases with an increase in the distancefrom the worm wheel 72. In other words, when the forces of the assistsprings 64 for cylinders #1 to #4, which are shown in FIG. 9, are P#1 toP#4, the forces of the assist springs 64 are set up so thatP#1>P#2>P#3>P#4. The forces of the assist springs 64 can be changed bycausing the assist springs 64 to differ, for instance, in the wirediameter, the number of turns, and the coil diameter. The forces of theassist springs 64 can also be changed by installing the assist springs64 for the cylinders at different mounting angles and without having tochange the designs of the assist springs 64.

The assist spring 64 generates a force that resists the forces of thelost motion spring 60 and valve spring 62. Therefore, when the force ofthe assist spring 64 is increased for parts that are positioned awayfrom the worm wheel 72 and low in rigidity in relation to deformation inthe rotation direction, the torsion of the control shaft 40 can becontrolled. It is then possible to prevent the valve discs 12 in thecylinders from varying in the lift amount and valve opening/closingtiming due to control shaft deformation. To control the deformation ofthe control shaft 40, the load applied by the lost motion spring 60 maybe varied from one cylinder to another to ensure that the force of thelost motion spring 60 decreases with an increase in the distance fromthe worm wheel 72.

In the example shown in FIG. 9, a worm mechanism is provided at the endof the control shaft 40 for a four-cylinder engine. However, even whenthe worm mechanism is positioned between cylinders #2 and #3, thedeformation of the control shaft 40 can be controlled by causing theforce of the assist spring 64 to increase with an increase in thedistance from the worm mechanism.

In the second embodiment, the assist spring 64 is provided to apply aforce in opposition to the forces of the lost motion spring 60 and valvespring 62 as described above. This makes it possible to considerablyreduce the driving force for rotating the control shaft 40 as is thecase with the first embodiment. Further, the force of the assist spring64 increases with an increase in the distance from the worm wheel 72,which regulates the rotation position of the control shaft 40. It istherefore possible to inhibit the control shaft 40 from being deformedby the load applied by the lost motion spring 60 and valve spring 62.Consequently, it is possible to inhibit the lift amount and operatingangle of each cylinder from being varied and provide the same intake airamount for all cylinders. As a result, it is possible to avoiddrivability deterioration and output decrease.

Further, the deformation of the control shaft 40 can be controlled. Itis therefore possible to decrease the diameter and wall thickness of thecontrol shaft 40. This makes it possible to decrease the drive torque ofthe motor 66 and reduce the size of the engine.

FIG. 10 is a schematic diagram illustrating the first/second embodimentin which a lost motion spring 61 made of a torsion spring is usedinstead of the lost motion spring 60 made of a coil spring.

In the configuration shown in FIG. 10, the lost motion spring 61 ispositioned on the side of the oscillation arm 22 to penetrate throughthe control shaft 40. One end of the lost motion spring 61 is inengagement with a protrusion 22 a that is provided on the side of theoscillation arm 22, and the other end is in engagement with anengagement section 40 a that is provided on the control shaft 40.

The force of the lost motion spring 61 causes the oscillation arm 22 tolift up the roller contact surface 28 (works in the direction ofrotating the oscillation arm 22 counterclockwise in FIG. 10). Therefore,the configuration shown in FIG. 10 permits the lost motion spring 61 toexercise the same function as the lost motion spring 60 that is made ofa coil spring. In other words, the lost motion spring 61 ensures thatthe cam 54 is mechanically coupled to the roller contact surface 28 viathe cam contact roller 48 and slide roller 50.

As described earlier, the second embodiment controls the deformation ofthe control shaft 40 by changing the force of the assist spring 64 inaccordance with the distance from the worm wheel 72, which regulates therotation position of the control shaft 40. However, the deformation ofthe control shaft 40 occurs due to the resultant force that the valvespring 62, lost motion spring 60, and assist spring 64 apply in thedirection of control shaft rotation. Therefore, when the resultant forceis varied in accordance with the distance from the worm wheel 72 on anindividual cylinder basis, it is possible to inhibit the control shaft40 from deforming. In other words, when the resultant force that thevalve spring 62, lost motion spring 60, and assist spring 64 apply inthe direction of control shaft rotation is decreased with an increase inthe distance from the worm wheel 72, it is possible to inhibit thecontrol shaft 40 from being deformed in the rotation direction by theforces of the springs.

More specifically, the deformation of the control shaft 40 can becontrolled by changing the force of the lost motion spring 60 inaccordance with the distance from the worm wheel 72, which regulates therotation position of the control shaft 40. In such a situation, theforce of the lost motion spring 60 in the variable valve mechanism 10for each cylinder is set up so that the force of the lost motion spring60 decreases with an increase in the distance from the worm wheel 72. Asdescribed earlier, the forces of the valve spring 62 and lost motionspring 60 are applied to the control shaft 40 and oriented in the samerotation direction. The amount of control shaft deformation by theforces of the valve spring 62 and lost motion spring 60 increases withan increase in the distance from the worm wheel 72. Therefore, when theforce of the lost motion spring 60 is decreased with an increase in thedistance from the worm wheel 72, it is possible to control the torsionand other deformation of the control shaft 40.

The deformation of the control shaft 40 can also be controlled bychanging the force of the valve spring 62 in accordance with thedistance from the worm wheel 72, which regulates the rotation positionof the control shaft 40. In such a situation, the force of the valvespring 62 for each cylinder is set up so that the force of the valvespring 62 decreases with an increase in the distance from the worm wheel72. The amount of control shaft deformation by the forces of the valvespring 62 and lost motion spring 60 increases with an increase in thedistance from the worm wheel 72. Therefore, the torsion and otherdeformation of the control shaft 40 can be controlled by causing theforce of the valve spring 62 to decrease with an increase in thedistance from the worm wheel 72.

The forces of the lost motion springs 60 for the cylinders can be variedby causing the lost motion springs 60 to differ, for instance, in thewire diameter, the number of turns, and the coil diameter. The forces ofthe lost motion springs 60 can also be varied by configuring the lostmotion spring mount in such a manner that the amount of lost motionspring compression varies from one cylinder to another. When the lostmotion springs 61 are made of a torsion spring as indicated in FIG. 10,the forces of the lost motion springs 61 can be varied by variouslysetting the angle between the horizontal and the extension direction ofthe engagement section 40 a (this angle is indicated by the symbol θ1 inFIG. 10). More specifically, referring to the FIG. 10, the force of thelost motion spring 61 works to rotate the oscillation arm 22counterclockwise. Therefore, when the value of the angle θ1, whichindicates the position of the engagement section 40 a that engages witheach lost motion spring 61, is increased with an increase in thedistance from the worm wheel 72 as viewed in the length direction of thecontrol shaft 40, it is possible to ensure that the force of the lostmotion spring 61 decreases with an increase in the distance from theworm wheel 72. When the position of the engagement section 40 a isvaried as described above, it is possible to vary the forces of the lostmotion springs 61 without changing the designs of the lost motionsprings 61. When the amount of lost motion spring compression is variedfrom one cylinder to another or when the position of the engagementsection 40 b is varied, it is not necessary to furnish a plurality oflost motion springs 60, 61 that vary in the force. Consequently, thenumber of parts can be reduced. Further, when the lost motion springs60, 61 are to be installed, it is not necessary to perform a step forchoosing from a plurality of lost motion springs 60, 61 that vary in theforce.

Further, the forces of the valve springs 62 can be varied by causing thevalve springs 62 to differ, for instance, in the wire diameter, thenumber of turns, and the coil diameter. The forces of the valve springs62 can also be varied by inserting a valve spring sheet 63, which variesin thickness, underneath the valve springs 62 as indicated in FIG. 10.When, in this instance, the thickness of the valve spring sheet 63 isdecreased with an increase in the distance from the worm wheel 72 asviewed in the length direction of the control shaft 40, it is possibleto ensure that the force of the valve spring 62 decreases with anincrease in the distance from the worm wheel 72. When the forces of thevalve springs 62 are varied by using the valve spring sheet 63 asdescribed above, the forces of the valve springs 62 can be variedwithout changing the designs of the valve springs 62. Therefore, it isnot necessary to furnish a plurality of valve springs 62 that vary inthe force. Consequently, the number of parts can be reduced. Inaddition, valve spring installation can be carried out without having toperform a step for choosing from a plurality of valve springs 62 thatvary in the force.

When the force of at least one of the lost motion spring 60, valvespring 62, and assist spring 64 is varied in the length direction of thecontrol shaft 40, and the resultant force that the valve spring 62, lostmotion spring 60, and assist spring 64 apply in the direction of controlshaft rotation is decreased with an increase in the distance from theworm wheel 72, it is possible to inhibit the control shaft 40 from beingdeformed in the direction of rotation by the forces of the springs.

Third Embodiment

A third embodiment of the present invention will now be described. Thebasic configuration and operation of the third embodiment of thevariable valve mechanism 10 are the same as those of the firstembodiment, which has been described with reference to FIGS. 1 to 4.

FIGS. 11A, 11B, and 11C illustrate the variable valve mechanism 10according to the third embodiment. More specifically, FIG. 11A is a planview illustrating the variable valve mechanism 10. FIG. 11B is a sideview that is taken in the direction of arrow B in FIG. 11A to illustratethe variable valve mechanism 10. FIG. 11C is a cross-sectional view thatis taken along section C—C of FIG. 11B to illustrate essential parts ofthe variable valve mechanism.

The configuration shown in FIGS. 11A, 11B, and 11C includes an internalcombustion engine cylinder head 80. The cylinder head 80 retains thecontrol shaft 40 via a control shaft bearing (not shown) and allows thecontrol shaft 40 to rotate. The essential parts of the variable valvemechanism 10, which have been described with reference to FIGS. 1 and 2and are not shown in FIGS. 11A, 11B, and 11C, are provided near thecylinder head 80. The internal combustion engine according to thepresent embodiment includes a plurality of in-line type cylinders (it ishereinafter assumed that the internal combustion engine according to thepresent embodiment includes four cylinders). The control shaft 40 ispositioned over the four cylinders.

A first gear 84, which is a spur gear, is fastened to the end of thecontrol shaft 40. A second gear 86, which is also a spur gear, is inengagement with the first gear 84. A rotation shaft 88 is fastened tothe center of the second gear 86. As shown in FIG. 11B, a semicircularworm wheel 90 is fastened to the rotation shaft 88. The worm wheel 90overlaps the second gear 86. The rotation shaft 88 is retained by thecylinder head 80 in a rotatable manner. When this configuration isemployed, the semicircular worm wheel 90 and the second gear 86, whichis shaped like a spur gear, can rotate on the rotation shaft 88 whilethe relative rotation angle between them is kept constant.

The motor 66, which functions as an actuator for rotating the controlshaft 40, is mounted on the side of the cylinder head 80. A worm gear94, which meshes with the aforementioned worm wheel 90, is fastened to arotation shaft for the motor 66. As indicated in the figures, thelateral surface of the worm gear 94 is provided with a spiral geargroove. The worm wheel 90 is provided with an inclined gear groove thatmeshes with the spiral gear groove.

The rotation shaft for the motor 66 is positioned 90 degrees from therotation shaft 88 for the worm wheel 90. The worm gear 94 and worm wheel90 can transmit the output torque of the motor 92 to the rotation shaft88 although their rotation shafts are not in alignment. Within theconfiguration shown in FIGS. 11A, 11B, and 11C, the torque transmittedto the rotation shaft 88 is transmitted to the control shaft 40 via thesecond gear 86 and first gear 84. Therefore, when this configuration isemployed, the rotation of the control shaft 40 can be controlled bycontrolling the rotation of the motor 66.

In the variable valve mechanism according to the present embodiment, therotation position of the control shaft 40 is adjusted within apredetermined angular range. Therefore, the gear mechanism connected tothe control shaft 40 should be capable of operating the control shaft 40within such an angular range. In the configuration according to thepresent embodiment, such an angular range can be sufficiently covered byrotating the worm wheel 90 through 180 degrees. In the presentembodiment, therefore, the worm wheel 90 is shaped like a semicircle tominimize the unnecessary portion contained in the gear mechanism.

Further, the variable valve mechanism according to the presentembodiment includes an assist spring 96, which is provided in the gearmechanism for transmitting the torque of the motor 66 to the controlshaft 40. The assist spring 96 is made of a coil spring, which ispositioned around the rotation shaft 88 for the worm wheel 90. One endof the assist spring 96 is fastened to the second gear 86 and the otherend is fastened to the cylinder head 80.

The assist spring 96 can generate an assist torque around its centralaxis. In the configuration described above, the assist spring 96 cangive a torque, which is oriented in a predetermined direction, to thesecond gear 86, rotation shaft 88, and worm gear 90. The rotation of therotation shaft 88 is transmitted to the control shaft 40 so that theintake valve lift amount changes. When rotation in one direction occurs,the lift amount increases. When rotation in another direction occurs,the lift amount decreases. In the present embodiment, the assist spring96 is installed so as to generate the assist torque in the direction ofincreasing the lift amount.

As described above, the variable valve mechanism according to thepresent embodiment is configured so that the motor 66 drives the controlshaft 40 via the gear mechanism that includes the worm wheel 90 and wormgear 94. The gear mechanism incorporates the assist spring 96 forimparting an assist torque, which is oriented in the great liftdirection, to the control shaft 40. Further, the assist torque isdirectly applied to the worm wheel 90.

When a combination of the worm wheel 90 and worm gear 94 is used, it ispossible to provide high normal efficiency and low inverse efficiency.Therefore, the variable valve mechanism according to the presentembodiment permits the torque generated by the motor 66 to betransmitted to the control shaft 40 with high efficiency. Further, thevariable valve mechanism prevents the torque input to the control shaft40 from being transmitted to the motor 66. Therefore, the variable valvemechanism can accurately control the rotation position of the controlshaft 40 by controlling the motor 66.

In the variable valve mechanism according to the present embodiment, theinfluence of an external force for rotating the control shaft 40 in thesmall lift direction, that is, the influence of the reactive force ofthe valve spring 62 and the force exerted by the lost motion spring 60,can be mitigated with the aforementioned assist torque. If the controlshaft 40 is to be rotated in the great lift direction in a situationwhere the assist torque does not exist, it is necessary to rotate thecontrol shaft 40 in resistance to various mechanical friction forces,the reactive force of the valve spring 62, and the like. In thisinstance, it is demanded that the motor 66 generate a great torque. As aresult, great electrical power is required for driving the motor 66, andthe gear mechanism and control shaft 40 are likely to twist.

If, on the other hand, the influence of the reactive force of the valvespring 62 and the like can be mitigated with the assist torque, thecontrol shaft 40 can be rotated in the great lift direction with a smallmotor torque. Consequently, when compared to a situation where theassist spring 96 does not exist, the variable valve mechanism accordingto the present embodiment is advantageous in that it, for example,reduces the size of the motor 66, decreases the power consumption fordriving the control shaft 40, and reduces the torsion of the controlshaft and the like.

Further, the configuration according to the present embodiment permitsthe control shaft 40 in a stationary state to smoothly start rotatingbecause the assist torque is directly applied to the worm wheel 90. Thereason will now be described with reference to FIG. 12. FIG. 12illustrates the relationship between the normal efficiency of the gearmechanism (the efficiency of torque transmission from the worm gear 94to the worm wheel 96), which comprises the worm gear 94 and worm wheel90, and their instantaneous rotation speed. More specifically, the curveindicated by a one-dot chain line in FIG. 12 represents the normalefficiency that prevails when the assist torque is not applied to theworm wheel 90. The curve indicated by a solid line in FIG. 12 representsthe normal efficiency that prevails when the assist torque, which isoriented in the direction of providing rotation assistance, is appliedto the worm wheel 90.

The coefficient of static friction between the worm gear 94 and wormwheel 90 is sufficiently greater than the coefficient of static frictionbetween the spur gears. If the motor 66 generates a torque that isoriented in the great lift direction while a force oriented in the smalllift direction is applied to the control shaft 40, a great load isimposed between the worm gear 94 and worm wheel 90 due to thecombination of the forces. Therefore, if the assist torque does notexist between the worm gear 94 and worm wheel 90, a great staticfriction force arises. As a result, the normal efficiency is remarkablylow in a region where the instantaneous rotation speed is near zero, asindicated by a one-dot chain line in FIG. 12. When the instantaneousrotation speed is increased to avert the influence of the staticfriction coefficient, the normal efficiency steadily remains high.

When an assist torque oriented in the great lift direction is applied tothe worm wheel 90, the force that is oriented in the small liftdirection and input to the control shaft 40 can be offset by the assisttorque. As a result, the static load imposed between the worm wheel 90and worm gear 94 can be rendered small. When the load is small, thestatic friction arising between the worm wheel 90 and worm gear 94 isalso small. Consequently, the normal efficiency within a lowinstantaneous rotation speed range is remarkably improved as indicatedby a solid line in FIG. 12. When the normal efficiency in such a rangeis improved, the control shaft 40 smoothly begins rotating in the greatlift direction. Thus, the control accuracy of the control shaft 40increases.

As described above, the assist spring 96 in the variable valve mechanismaccording to the present embodiment permits the control shaft 40 tosmoothly rotate in the great lift direction with a small motor torque.Further, an external force, which is oriented in the small liftdirection, is originally applied to the control shaft 40. Thus, when thecontrol shaft 40 is moved in the small lift direction, a good operationcharacteristic is inevitably achieved. In the variable valve mechanismaccording to the present embodiment, therefore, the control shaft 40 canbe smoothly rotated in any direction even when a small force is applied.

The third embodiment, which has been described above, assumes that theassist spring 96 is incorporated in the gear mechanism for rotating thecontrol shaft 40 in order to change the operating angle and lift amountof the valve disc 12. However, the present invention is not limited tosuch a configuration. More specifically, a mechanism for changing theoperating angle and lift amount of the valve disc 12 by moving thecontrol shaft 40 in axial direction may be employed so that an assistspring for generating an assist torque in the great lift direction isincorporated in a gear mechanism for transmitting a driving force to thecontrol shaft 40.

As is the case with the first embodiment, the third embodiment, whichhas been described above, assumes that the lost motion spring 60 andvalve spring 62 both generate a force for changing the variable valvemechanism 10 in the small lift direction. However, the present inventionis not limited to such a configuration. The present invention is alsoapplicable to a mechanism in which the lost motion spring generates aforce in the great lift direction.

The third embodiment, which has been described above, assumes that thevariable valve mechanism 10 changes both the operating angle and liftamount in accordance with the rotation position of the control shaft 40.However, the present invention is not limited to such a configuration.More specifically, the variable valve mechanism may change either theoperating angle or lift amount. In such a situation, the same advantagesare obtained as in the third embodiment when an assist spring isprovided to generate a force for moving the control shaft, which changesonly the valve disc operating angle, in the great operating angledirection or generate a force for moving the control shaft, whichchanges only the valve disc lift amount, in the great lift direction.

In the third embodiment, which has been described above, the first armmember 24 and second arm member 26 correspond to the “adjustmentmechanism” according to the seventh aspect of the present invention; themotor 66 corresponds to the “actuator” according to the seventh aspectof the present invention; the worm gear 94, worm wheel 90, second gear86, and first gear 84 correspond to the “gear mechanism” according tothe seventh aspect of the present invention; and the assist spring 96corresponds to the “assist force generation means” according to theseventh aspect of the present invention.

When, as is the case with the second embodiment, the third embodimentvaries the force of the lost motion spring 60 or valve spring 62 in thelength direction of the control shaft 40, and ensures that the resultantforce applied in the direction of control shaft rotation by the valvespring 62 and lost motion spring 60 decreases with an increase in thedistance from the first gear 84, it is possible to inhibit the controlshaft 40 from being deformed in the rotation direction by the forces ofthe springs. For example, when the force of the lost motion spring 60decreases with an increase in the distance from the first gear 84, it ispossible to control the torsion of the control shaft.

Fourth Embodiment

A fourth embodiment of the present invention will now be described withreference to FIGS. 13A and 13B. FIGS. 13A and 13B illustrate alubricating oil flow path that is used in a variable valve mechanismaccording to the fourth embodiment of the present invention. Morespecifically, FIG. 13B is an enlarged cross-sectional view illustratingthe engagement between the worm gear 94 and worm wheel 90. FIG. 13A is across-sectional view that is taken along section A—A of FIG. 13B toillustrate the variable valve mechanism according to the presentembodiment. It is assumed that FIGS. 13A and 13B indicate the up-downpositional relationship that prevails when the internal combustionengine is mounted in a vehicle.

The variable valve mechanism according to the present embodiment issubstantially the same as the variable valve mechanism according to thethird embodiment except that the former includes a lubricating oil flowpath, which is described below. For the sake of convenience, thevariable valve mechanism according to the present embodiment isconfigured so that the worm wheel 90 is fully circular and directlyfastened to the control shaft 40. However, such a configuration is notessential to the present invention. The mechanism according to thepresent embodiment is characterized by the fact that it includes alubricating oil flow path, which is described below. Elements that areshown in FIGS. 13A and 13B and similar to previously described elementsare assigned the same reference numerals as the previously describedelements and will be briefly described or will not be described at all.

As shown in FIG. 13B, the motor 66 in the variable valve mechanismaccording to the present embodiment is fastened to the cylinder head 80.The internal space of the cylinder head 80 is hermetically sealed by ahead cover 100, which is installed over the internal space. A space 102that is shaped to match the outline of the worm gear 94 and a space 104that is shaped to match the outline of the worm wheel 90 are formedwithin the cylinder head 100. These spaces 102 and 104 are integral witheach other. The worm gear 94 and worm wheel 90 are housed economicallyin these spaces.

The top of space 104 in which the worm wheel 90 is housed communicateswith an oil supply path 106. The oil supply path 106 is used during aninternal combustion engine operation so that lubricating oil forciblyfed from an oil pump is partly introduced into spaces 102 and 104. Anoil seal 108 is installed over the rotation shaft of the motor 66 tosurround the rotation shaft and isolate space 102 from the externalspace. Further, as indicated in FIG. 13A, another oil seal 110 isinstalled over the control shaft 40 to surround the control shaft 40 andisolate spaces 102 and 104 from the external space. Therefore, spaces102 and 104 are filled with the lubricating oil during an internalcombustion engine operation.

As shown in FIG. 13A, an oil flow path 112, which is extended in axialdirection, is formed inside the control shaft 40. The end of the oilflow path 112 is sealed with a seal plug 114. The control shaft 40 isprovided with an oil supply hole 116, which permits the oil flow path 92to communicate with spaces 102 and 104. During an internal combustionengine operation, therefore, the lubricating oil, which fills spaces 102and 104, is supplied to the oil flow path 112 via the oil supply hole116.

The cylinder head 80 includes bearings 118. The bearings 118 areprovided on both sides of the internal combustion engine cylinders toretain the control shaft 40. The control shaft 40 is retained by thesebearings 118 in a rotatable manner. Essential parts of the variablevalve mechanism 10, which correspond to each cylinder, are installedover the control shaft 40 at a position that is sandwiched between twobearings 118. More specifically, the two oscillation arms 22 and onenon-oscillation section 36, which are included in the variable valvemechanism 10, are installed over the control shaft 40 at a position thatis sandwiched between the two bearings 118.

The control shaft 40 is provided with an oil supply hole 120, which isconnected to the oil flow path 112. The oil supply hole 120 is providedat a position that corresponds to each bearing 118, each oscillation arm22, and each non-oscillation section 36. The non-oscillation section 36is provided with an oil flow path 122. One end of this oil flow path 122is connected to the oil supply hole 120 and the other end is connectedto the side of the rotation shaft 46 of the oscillation roller section38. Therefore, the lubricating oil flowing inside the control shaft 40is supplied to each lubricating point via the oil supply hole 120, oilflow path 122, and the like.

The lubricating oil flowing to the oil flow path 112 of the controlshaft 40 from spaces 102 and 104 in the variable valve mechanismaccording to the present embodiment is subsequently collected in an oilbasin inside the internal combustion engine via various lubricatingpoints and the like. When the internal combustion engine stops to shutoff the new lubricating oil supply to spaces 102 and 104 from the oilsupply path 106, the lubricating oil flow to the oil flow path 112 stopsin the course of time, thereby terminating the lubrication oilcirculation.

As regards the lubricating oil flow path shown in FIGS. 13A and 13B, thelubricating oil flowing into spaces 102 and 104 does not flow out ofspaces 102 and 104 until it passes through the oil supply hole 116 andflows to the oil flow path 112. The oil supply hole 116 is positionedhigher than the engagement between the worm gear 94 and worm wheel 90.Therefore, the lubricating oil level within spaces 102 and 104 ismaintained at a position higher than the engagement between the wormgear 94 and worm wheel 90 even while the internal combustion engine isstopped.

Under the above conditions, the lubricating oil can always be suppliedabundantly between the worm gear 94 and worm wheel 90. Even when thelubricating oil does not sufficiently circulate, for instance,immediately after internal combustion engine startup, the variable valvemechanism according to the present embodiment can transmit the outputtorque of the motor 66 efficiently to the control shaft 40.

INDUSTRIAL APPLICABILITY

As described above, the variable valve mechanism according to thepresent invention makes it possible to reduce the drive load on thecontrol shaft that changes the valve disc lift amount and operatingangle. It can be effectively used to exercise various variable valvemechanism functions within an internal combustion engine.

1. A variable valve mechanism for changing lift amount and operatingangle of an internal combustion engine valve disc, the variable valvemechanism comprising: a first cam, which rotates in accordance withcrankshaft rotation; a transmission member that includes a second cam,which oscillates in synchronism with the rotation of the first cam andtransmits a force exerted by the first cam to the valve disc; a controlshaft, which is adjusted for a predetermined rotation position, thecontrol shaft having the transmission member pivotally engaged thereon;an adjustment mechanism for varying the lift amount and operating angleof the valve disc by changing an oscillation range of the transmissionmember in accordance with the rotation position of the control shaft; alost motion spring for pressing the transmission member toward the firstcam to ensure that the transmission member remains coupled to the firstcam; and an assist spring for pressing the transmission member inresistance to a force exerted by the lost motion spring.
 2. The variablevalve mechanism according to claim 1, wherein the lost motion springpresses the transmission member in a direction of changing the liftamount and operating angle of the valve disc from a great lift/greatoperating angle side to a small lift/small operating angle side; andwherein the force exerted on the transmission member by the assistspring increases with a decrease in the lift amount and operating angleof the valve disc.
 3. The variable valve mechanism according to claim 1,further comprising a valve spring for pressing the valve disc toward thetransmission member, wherein the assist spring presses the transmissionmember in resistance to the valve spring's force that is exerted on thetransmission member via the valve disc.
 4. The variable valve mechanismaccording to claim 3, further comprising: an actuator for generating adriving force for changing the rotation position of the control shaft;and a gear mechanism that is positioned between the actuator and thecontrol shaft; wherein a plurality of the transmission members, whichare provided for the valve discs of various cylinders, are coupled tothe common control shaft; wherein the force exerted by the lost motionspring, the force exerted by the assist spring, and the force exerted bythe valve spring are transmitted in the rotation direction of thecontrol shaft via the transmission member and the adjustment mechanism;and wherein the resultant force applied in the rotation direction of thecontrol shaft by the lost motion spring, the assist spring, and thevalve spring decreases with an increase in the distance from the gearmechanism as viewed in a length direction of the control shaft.
 5. Thevariable valve mechanism according to claim 4, wherein a force exertedon the transmission member by the assist spring increases with anincrease in the distance from the gear mechanism as viewed in the lengthdirection of the control shaft.
 6. The variable valve mechanismaccording to claim 4, wherein the force exerted on the transmissionmember by the lost motion spring decreases with an increase in thedistance from the gear mechanism as viewed in the length direction ofthe control shaft.
 7. A variable valve mechanism, which is capable ofchanging operating angle and/or lift amount of an internal combustionengine valve disc, the variable valve mechanism comprising: a controlshaft whose status is controlled to change the operating angle and/orlift amount; an oscillation arm that is positioned between a cam and thevalve disc to oscillate in synchronism with cam rotation and transmit aforce exerted by the cam to the valve disc, the oscillation armpivotally engaged on the control shaft; an adjustment mechanism forchanging a basic relative angle of the oscillation arm relative to thevalve disc in accordance with a status of the control shaft; an actuatorfor generating a driving force for changing the status of the controlshaft; a gear mechanism that is positioned between the actuator and thecontrol shaft; and assist force generation means for applying an assistforce to the gear mechanism in order to increase the operating angleand/or lift amount.
 8. The variable valve mechanism according to claim7, wherein the gear mechanism includes a worm wheel and a worm gear,which are interconnected so as to position the worm gear toward theactuator and the worm wheel toward the control shaft; and wherein theassist force is applied to the worm wheel or to a structure integralwith the worm wheel.
 9. The variable valve mechanism according to claim7, further comprising a lost motion spring for pressing the oscillationarm toward the cam to ensure that the oscillation arm remainsmechanically coupled to the cam, wherein the oscillation arm moves in adirection of increasing the amount of lost motion spring deformationwhen the generation of a great operating angle and/or lift amount isrequested.
 10. The variable valve mechanism according to claim 9,wherein a plurality of the oscillation arms provided for the valve discsof various cylinders are coupled to the common control shaft; andwherein the force exerted by the lost motion spring decreases with anincrease in the distance from the gear mechanism as viewed in a lengthdirection of the control shaft.
 11. A variable valve mechanism forchanging lift amount and operating angle of an internal combustionengine valve disc, the variable valve mechanism comprising: a first cam,which rotates in accordance with crankshaft rotation; a transmissionmember that includes a second cam, which oscillates in synchronism withthe rotation of the first cam and transmits a force exerted by the firstcam to the valve disc; a control shaft, which is adjusted for apredetermined rotation position, the control shaft having thetransmission member pivotally engaged thereon; an adjustment mechanismfor varying the lift amount and operating angle of the valve disc bychanging an oscillation range of the transmission member in accordancewith the rotation position of the control shaft; a lost motion springfor pressing the transmission member toward the first cam to ensure thatthe transmission member remains coupled to the first cam; and an assistspring for generating a force that resists a force exerted by the lostmotion spring.